The present invention pertains to cellular telecommunications, and particularly to synchronization of nodes in a cellular network such as a code division multiple access (CDMA) cellular network. In particular, certain embodiments of this invention relate to a system and corresponding method for adjusting the oscillator of a slave node in order to avoid and/or reduce potential phase jumps or steps associated with directly adjusting a frame counter of that slave node.
This application is related to commonly owned U.S. patent application Ser. No. 09/095,585, filed Jun. 11, 1998 (atty. ref. 2380-5), U.S. Ser. No. 09/257,233, filed Feb. 25, 1999 (atty. ref. 2380-94), and U.S. Ser. No. 09/443,208, filed Nov. 18, 1999 (atty. ref. 2380-113), the disclosures of which are all hereby incorporated herein by reference.
In mobile telecommunications, a mobile station (MS) such as mobile cellular telephone, communicates over radio channels with base station(s) (BS or BTS). Typically a plurality of base stations are connected by an upper node, such as a radio network controller (RNC), to a mobile switching center (MSC). The mobile switching center (MSC) is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network (PSTN).
In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a code (such as channelization and/or scrambling codes) to distinguish it from information for other mobile stations which are utilizing the same radio frequency band(s). Thus, in CDMA, individual radio links are discriminated on the basis of codes.
In CDMA systems, on the downlink (e.g., from the base station(s) to a MS) the same baseband signal with suitable codes is typically sent from several base stations with overlapping coverage at approximately the same time to a particular MS(s). In other words, frames with equal user data are sent from different base stations at approximately the same time on the downlink to the mobile station. The MS can thus receive and use signals from several base stations simultaneously. Since the radio environment changes rapidly, an MS likely has radio channels to several base stations at the same moment, e.g., so that the MS can select or combine the best channel and, if necessary, use signals directed to the MS from various base station(s) in order to keep radio interference low and capacity high. This selection procedure in a MS among frames from different base stations enables optimization of the quality of the MS-BS connection.
As for the uplink (e.g., from the MS to BS(s)), user data sent in frames from the MS is often received in multiple BSs. Frame identities are typically attached to uplink frames that are received by base station(s), so as to enable selection by an upper node (e.g., RNC) of one of a plurality of signals received from an MS at approximately the same time at different base stations. While interference is kept low, the BS having the best quality reception of the signal of the MS may randomly change during the course of the connection. Therefore, an upper node such as a radio network controller (RNC) may select the best quality one(s) of the comparable number of frames received from the mobile station by the different base stations diversity combining.
This utilization of radio channels between multiple base stations and a MS in a CDMA scheme, as summarized above, is known as xe2x80x9csoft handoverxe2x80x9d in that different frames may be selected as a function of reception quality, strength, or the like.
The principles of diversity combining and soft-handover require that the base stations participating in a particular connection be synchronized relative to the upper node, e.g., to the RNC. Synchronization is required since, among other things, the plural base stations participating in a connection must send the same frame information at the same time to the MS involved in the connection.
A way to identify frames in the downlink and uplink directions is to attach a sequence number to each frame. In the downlink, sequence numbers can be correlated in the base stations against a reference timing/numbering in order to align frames to intended transmission time (air interface). In the uplink, sequence numbers (related to base station reference timing/numbering) can be attached to frames in each base station before transfer to the RNC (where diversity combining/selection is done based on these numbers).
As can be seen from the above, there exists a need in the art for efficient alignment of timing in different base stations and/or RNC(s) in order to enable reliable soft handover operations.
Typically, timing systems at nodes such as RNCs and BSs have frame counters driven by respective oscillators. Unfortunately, oscillators tend to experience xe2x80x9cdriftxe2x80x9d over time (e.g., how many seconds, or fractions of seconds, they drift over a month or year). Drift may be caused by aging, temperature, voltage instability, or the like. Drift is unitless and tends to be described in terms of xe2x80x9cppmxe2x80x9d (parts per million) or xe2x80x9cppbxe2x80x9d (parts per billion). For example, 20 ppb means that the frequency uncertainty is +/xe2x88x9220 cycles per one billion nominal cycles. Oscillator drift is one reason why timing systems in respective BS nodes need to be periodically adjusted so as to fulfill air-interface radio requirements and phase drifting requirements so as to stay in sync with a master timing unit such as at a controlling RNC node.
In some systems such as WCDMA or DS-CDMA cellular networks, an upper node (e.g., RNC) has a master system frame counter which is locked to an external reference or clock source. It is desirable for such systems to have a procedure capable of synchronizing base stations so that base stations are substantially phase stable in time (i.e., to avoid substantial phase drifting by base stations). Avoiding/reducing substantial BS phase drift reduces the likelihood of BSs drifting apart in phase during a connection with a particular MS. In other words, it is desirable to keep BSs phase stable enough to prevent one or more BSs from substantially drifting apart during a connection with a MS (when the MS is connected via several BSs). It is thus desirable to achieve a substantially common frame counter and/or a substantially phase stable frame counter in the entire cellular system to make it easier to determine offset values which are to be used in securing radio frames. In situations where a network (e.g, WCDMA network) need not have an absolute phase in the entire system but instead desires substantially phase stable nodes, only substantially phase stable frame counter(s) are desired. This achieves short delays and ensures that the same frames are sent on the downlink in macro-diversity and that the same frames are combined on the uplink in a diversity handover unit (DHT).
It is known that frame counters in base stations may need to be adjusted periodically. In a CDMA system where each user channel includes a number of chips per coded information bit (different numbers of chips may be used depending upon the spreading factor used and/or on the number of users), the chip sequence must be adjusted whenever the frame counter is adjusted. Unfortunately, as discussed more fully below, phase jumps tend to occur upon directly adjusting a frame counter which in turn can lead to dropped call(s).
Commonly owned WO 99/33207 discloses a synchronization system for a cellular telecommunications network. A master timing unit is provided at a control node and a slave timing unit at a base station. When it is determined that an adjustment is necessary, the master timing unit transmits a synchronization adjustment signal to the slave timing unit so that the xe2x80x9ccounterxe2x80x9d at the base station is adjusted accordingly, while keeping the oscillator at the base station untouched.
Unfortunately, the direct frame counter adjustment in WO 99/33207 is problematic in that it can lead to hardware phase adjustment problems. For example, direct adjustment of the counter can often lead to phase jumps/steps which negatively impact the air-interface (e.g., calls could be dropped). In other words, when adjusting the frame counter directly it can be tricky to adjust all logic to the new desired phase in a disturbance-free manner (e.g., algorithms may have problems finding the new phase).
What is needed therefore, and an object of the present invention, is an accurate and reliable technique for synchronizing timing units, such as timing units at base stations, with an upper node in a manner so as to avoid the potential hardware phase adjustment problems (e.g., phase jumps/steps) of the frame counter adjustment method of WO 99/33207.
Synchronization is effected in a cellular telecommunications network between a timing unit located at control node (e.g., RNC) of the network and a slave timing unit (STU) located at a controlled node (e.g., BS) of the network. Upon determining that a synchronization adjustment of the slave timing unit is necessary, an adjustment signal (e.g., voltage signal) is caused to be input to the oscillator of the slave timing unit in order to change the frequency of the slave""s oscillator. Frame counter(s), in communication with the oscillator at the slave timing unit, will thus follow the oscillator continuously with smooth phase adjustments. By avoiding a direct adjustment of the frame counter(s), undesirable phase jumps or steps are avoided that can potentially disturb the air-interface between a base station and mobile station (MS). Thus, a goal of achieving a substantially phase stable frame counter(s) can be realized, and in certain embodiments the likelihood of a BS drifting apart in phase relative to another BS connected to the same MS is reduced.
According to one exemplary and non-limiting embodiment, an initiating one of the master timing unit and the slave timing unit transmits a synchronization analysis command message including a first parameter (e.g., t1) to a responding one of the master timing unit and the slave timing unit. In response, the responding timing unit sends a synchronization analysis response message which includes at least second parameter (e.g., t2) and preferably a third parameter (e.g., t3) to the initiating timing unit. The initiating timing unit uses e.g., parameters extracted from the synchronization analysis response message to determine a synchronization adjustment value for the slave timing unit. In certain embodiments, the master timing unit transmits the synchronization adjustment value in a synchronization adjustment command message to the slave timing unit. The slave timing unit calculates and performs the adjustment by inputting an adjustment signal (e.g., a voltage signal) to the oscillator of the slave timing unit, and then notifies the master unit.
In one exemplary non-limiting embodiment, the first parameter included in the synchronization analysis command message may be a first time stamp value t1 related to the time that the synchronization analysis command message is transmitted from the initiating timing unit to the responding timing unit. The second parameter, inserted in the synchronization analysis response message by the responding timing unit, may be a second time stamp value t2 related to the time that the synchronization analysis command message is received at the responding timing unit. The third time stamp value t3, also inserted in synchronization analysis response message by the responding timing unit, may be related to the time that the synchronization analysis response message is sent from the responding timing unit. The initiating unit determines a fourth time stamp value t4 indicative of a time of reception of the synchronization sequence response message at the initiating timing unit.
In an exemplary embodiment where the initiating unit is the master unit, the initiating timing unit may determine the synchronization adjustment value by comparing the second parameter t2 included in the synchronization analysis response message with a predicted second parameter t2-predicted. The predicted second time stamp value t2-predicted may be determined as t2-predicted=((t1+t4)/2)xe2x88x92((t3xe2x88x92t2)/2). The synchronization adjustment value may then be determined as t2-predictedxe2x88x92t2. However, when the initiating unit is the slave timing unit, the initiating timing unit may determine the synchronization adjustment value by comparing the first parameter t1 included in the synchronization analysis response message with a predicted first parameter t1-predicted. The predicted first time stamp value t1-predicted may then be determined as t1-predicted=((t2+t3)/2)xe2x88x92((t4xe2x88x92t1)/2). The synchronization adjustment value may then be determined as t1-predictedxe2x88x92t1. The parameters t1 through t4 are preferably values of system frame counters. In particular, the parameters t1 and t4 are then-current values of a system frame counter of the initiating timing unit. The parameters t2 and t3 are then-current values of a system frame counter of the responding timing unit. The synchronization adjustment value is used to adjust the frequency of the oscillator of the timing unit at, e.g., the slave node (e.g., BS).
In different embodiments, the master timing unit can be located in a control node such as an RNC, or even in a mobile switching center (MSC). The slave timing unit can be located in a BS, or alternatively in the control node (such as in a diversity handover unit situated at the control node). Thus, the aforesaid explanation(s) of time stamps applies to different embodiments of this invention, regardless of whether or not the master timing unit is in a control node (e.g., RNC) or controlled node (e.g., BS), and regardless of whether or not the initiating node is the master timing unit, a slave timing unit, at a control node, and/or at a controlled node. Accordingly, a hierarchy is possible of master timing units (MTUs) and slave timing units (STUs) (e.g., an exemplary hierarchy could be MSC-RNC1, RNC1-RNC2, RNC2-BS, etc.).
Accordingly, certain embodiments of the instant invention result in one or more of the following advantage(s). First, by directly adjusting the oscillator instead of the frame counter (e.g., at the BS), undesirable phase jumps or steps can be avoided or reduced so as to maintain an adequate air-interface. Second, certain embodiments enable oscillator(s) to be adjusted automatically, without maintenance personnel having to physically visit BS sites for that reason (i.e., manual oscillator adjustments can be reduced and/or avoided). Third, manual oscillator adjustments can be avoided or reduced for BS oscillators that are locked to a transport network. Fourth, the clock in the BS is adjusted without having to rely solely on the transport network which can experience clock uncertainty. Fifth, the synchronization system/method is decoupled from the transport network principle (e.g., ATM, IP, or FR), where the availability of a network synchronization reference in some networks is/are questionable. Sixth, phase drifting in nodes such as base stations can be reduced and/or corrected, and thus the likelihood of a base station (one of a plurality of BSs on that connection with a particular MS) drifting apart in phase from other base stations on a given connection is reduced.